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The MCQs below are drawn from the Physics subject category.
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861
Doppler effect is seen in
A. Sound waves
B. Light waves
C. Both a and b
D. None of these
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Correct Option
Explanation
The Doppler effect occurs for both sound and light waves when there is relative motion between source and observer, so option C is correct.
862
A toothed wheel is rotating at 120 r.p.m and a postcard is placed against its teeth. How many teeth must it have to produce a note whose pitch is same as that of a tuning fork of frequency 256 per second?
A. 120
B. 256 × 120
C. 128
D. 256
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Correct Option
Explanation
120 r.p.m = 2 revolutions per second, so pulses per second = 2 × N; setting 2N = 256 gives N = 128 teeth.
863
When the length of a simple pendulum is doubled, find the ratio of the new frequency to the old frequency:
A. √2
B. 1/4
C. 1/2
D. 1/√2
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Correct Option
Explanation
Pendulum frequency f ∝ 1/√L, so doubling length reduces frequency by factor 1/√2 (new/old = 1/√2).
864
If the pressure amplitude in a sound wave is tripled, then by what factor is the intensity of the sound wave increased?
A. √5
B. 6
C. 9
D. 3
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Correct Option
Explanation
Intensity is proportional to the square of pressure amplitude, so tripling amplitude increases intensity by 3^2 = 9.
865
A stretched string of length 40 cm produces a note of frequency 200 vibrations per second. What length of the same string at the same tension will produce a note of 500 vibrations per second?
A. 24 cm
B. 16 cm
C. 12 cm
D. 36 cm
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Correct Option
Explanation
For a stretched string at same tension f ∝ 1/L, so L2 = L1*(f1/f2) = 40*(200/500) = 16 cm.
866
For a resonance tube the air columns for the first and the second resonances differ by 31.5 cm. The wavelength of the sound waves in the tube is
A. 63.0 cm
B. 2 m
C. 252.0 cm
D. 31.5 cm
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Correct Option
Explanation
Successive resonances differ by λ/2, so λ = 2*(31.5 cm) = 63.0 cm.
867
What is the title for combining of notes that produce jarring effect on the ear?
A. Noise
B. Harmony
C. Discord
D. Melody
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Correct Option
Explanation
A jarring or unpleasant combination of notes is called a discord.
868
The velocity of sound in a gas in which two waves of wavelengths 50 cm and 50.5 cm produce 6 beats per second is
A. 228 m s^-1
B. 296 m s^-1
C. 303 m s^-1
D. 332 m s^-1
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Correct Option
Explanation
Beat frequency = v|1/λ1 − 1/λ2| ≈ v*(0.005/(0.5×0.505)) ≈ 0.019802 v; so v = 6/0.019802 ≈ 303 m/s.
869
When a sound wave goes from air into water, the quantity that remains unchanged is its:
A. Speed
B. Amplitude
C. Some duplicate option
D. Frequency
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Correct Option
Explanation
Frequency remains the same across media boundaries; speed and wavelength change.
870
We can hear beats when the difference in the frequencies of two sounding bodies is not more than
A. 4
B. 2
C. 6
D. 10
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Correct Option
Explanation
Beats are typically audible when the frequency difference is small (commonly a few hertz); among choices, 6 Hz is the typical upper practical limit listed.
